Method and apparatus for coating metal strip and wire



Jan. 30, 1962 5, A. BARTHOLOMEW 3,019,125

METHOD AND APPARATUS FOR COATING METAL STRIP AND WIRE Filed March 24,1959 i INVENTOR & GEORGE A. BART/ 0L OMEW a Attorney United StatesPatent "cc 3,019,126 METHOD AND APPARATUS FOR COATING METAL STRIP ANDWIRE George A. Bartholomew, Penn Township, Allegheny County, Pa.,assignor to United States Steel Corporation, a corporation of New JerseyFiled Mar. 24, 1959, Ser. No. 801,626 3 Claims. (Cl. 117-17) Thisinvention relates to the coating of metal with a protective film and, inparticular, to the continuous coating of a product of indefinite lengthsuch as strip or wire.

The object of the invention is to provide a simple and effective methodand apparatus for applying to a traveling product a coating of materialin finely divided form.

A further object is to provide control means adapted to insure theformation of a coating of uniform thickness.

In general terms, my improved method comprises progressively heating theproduct to be coated and drawing it in strand form through a fluidizedbed of finely divided coating material. This material may be athermoplastic synthetic resin such as polyethylene, a refractorymaterial such as magnesium oxide or a metal, e.g., aluminum. I employmeans to heat the product to be coated immediately before it enters thefluidized bed, and after it emerges therefrom if desirable.Alternatively, I may heat the coating particles directly by thecombustion of fuel in the fluidized bed. I also provide automatic meansfor maintaining the proper depth of the fluidized bed and the degree offiuidization thereof. I may also utilize electrostatic force to aid theattraction of coating particles to the traveling wire or strip.

A complete understanding of the invention may be obtained from thefollowing detailed description and explanation which refer to theaccompanying drawings illustrating the present preferred embodiment. Inthe drawmgs:

FIGURE 1 is a flow diagram showing one form of apparatus schematicallyin vertical section; and

FIGURE 2 is a similar view showing a modification.

Referring now in detail to the drawings and, for the present, to FIGURE1, I provide apparatus for coating strip or wire comprising a preheatingchamber and a coating chamber 11, vertically disposed. The coatingchamber includes an enlarged upper portion 12 above which are mountedradiant or induction heating units 13. Metal strip S is unwound from acoil entering chamber 10, passes between contact rolls 14 then through afluidized bed 15 of coating-particles and, after passing between units13, passes between heated embossing rollers 16 and is then recoiled at17. The portion of the strip between upper and lower pairs of rolls 14is heated by any convenient means such as radiant heaters, induction orby electric current, to a temperature of from 120 F. to 660 F.Preferable deposition temperatures are specific to individual coatingcompounds. Deposition temperature ranges for several classes of coatingmaterials have been found to be:

Cellulosics About 300 F. Epoxys AboutlSO" F. Nylons About 630 F.Polyethylenes About 325 F.

3,019,125 Patented Jan. 30, 1962 ing a control valve 22. As thepreheated strip ascends through the fluidized bed, the particles ofcoating mate rial, being heated by contact with the strip, adherethereto and fuse together, forming a continuous coating of uniformthickness. The enlarged upper portion 12 of the coating chamber providesa settling space for any coating material which tends to follow thestrip without adhering to it. Heating elements 13 effect a final curingor smoothing of the coating by maintaining it above its flowtemperature. The coated strip passes between erabossing rolls 16 whilethe coating is still plastic and capable of being impressed with thepattern of the rolls. Thereafter the strip cools in the atmosphere to anontacky condition before being recoiled.

I provide means for automatically controlling the flow of fluidizing airto maintain a predetermined bulk density in bed 15. For this purpose, Iemploy a controller 23 such as Hagan Corporation's Ring Balance Meter"responsive to the pressure difierential between points P and P inchamber 11 and to an adjustable set-point control 24. Controller 23operates valve 22 which is a motorized valve such as the Flex Valve ofFarris Flexible Valve Corporation. A similar controller 25, responsiveto the pressure of the column of coating material in chamber 11,operates valve '19 of the same type as valve 22, in accordance with theadjustment of a set-point control 26.

The final coating thickness may be accurately controlled by regulationof five controllable factors: (a) strip-powder contact time, (b) preheattemperature, (c) bed density, (d) powder size, (e) flow characteristicsof the powder. Contact time may be controlled by strip speed andfluid-bed height. Preheat temperature, regulated to correspond with thethermal requirements of the coating powder, is a function of heatingcurrent and strip speed and may be readily controlled over a wide rangeof operational conditions. Bed density is a measore of the frequency ofparticle-strip collisions. In conjunction with strip temperature whichdetermines the adhesiveness of the coating particles and contact time,bed density is an effective control of coating thickness. Bed density,of course, is controlled by air-flow rate and bed depth, both of whichare measured directly and automatically controlled as just explained.

Instead of air for fluidizing, I may introduce hot combustion gases intothe bed 15 or a combustible mixture of fuel and air and effectcombustion in the chamber itself.

In coating strip with certain materials such as Teflon plastic(polytetrafiuoroethylene), Bakelite or other thermosetting resins, whichmight be adversely aflected by the heat of the strip, I prefer toeliminate the preheating of the strip. In that case, I effect initialadherence of the particles to the strip by electrostatic attraction andthen cause final adherence by subsequent treatment such as heating ofthe strip with the particles thereon. Apparatus for this process isshown in FIGURE 2 and is substantially the same as that of FIGURE 1except for the omission of preheating chamber 10. In lieu thereof, Iprovide entry contact rollers 27. A source 28 of high voltage directcurrent is connected between rollers 27 and electrodes 30 mounted inchamber 11 adjacent the strip path. As the strip passes upwardly throughchamber 11 at atmospheric temperature, the electrostatic force betweenthe strip and the particles of coating material causes them to beattracted tothe strip and adhere thereto. As the strip emerges from theupper portion 12 of the chamber 11, the adhering particles aremoderately heated by units 13 so as to fuse them into an adherentcoating.

The control of electrostatic deposition is dependent upon essentiallythe same variables as those controlling aorarae the thermodepositionprocess. Strip speed, air flow, bed depth and density, contact time areall of major importance. Voltage, however, replaces preheating as theprime deposition-control factor. It is more important to control thephysical and electrical properties of the powder here than in thethermodeposition process. However, except when plastic reflow isdesired, thermal prop erties of the coating materials are less importantin this process than in the thermodeposition process.

My process permits rapid, continuous application of accuratelycontrolled coatings. By heating the moving strip in an enclosed chamberonly a fraction of a second before coating, I effect a drastic reductionin thermal loss. By the continuous application of heat before, duringand after coating, I exercise complete control of strip tem perature.Immersion time of the strip in the bed is precisely regulated by the bedheight and strip speed, both of which are controlled in my process. Ifurther control the density of the fluid bed and, therefore, have anadditional processing variable which, with the alternate hotbed andelectrostatic-deposition methods, supplement process flexibility andfacilitate attaining the most advantageous deposition conditions.

Many advantageous coating materials such as magnesium oxide,diatomaceous earth and bentonite, are not plastic in the lowertemperature range (under 650 F.)

, and may require application temperatures in other processes so high asto be detrimental to the properties of the steel base. In my process,however, such materials may I be applied to steel strip without undueharm to the base metal. This is accomplished not by heating the stripand requiring thermal transfer to colliding particles, but rather by theapplication of heat directly to the fluidized particles. By fiuidizingwith a combination of a suitable fuel gas and air, actual combustion maybe maintained within the coating bed. The excellent agitation and mixingcharacteristics of such beds provide almost instantaneous transfer ofheat to the suspended particles, which may be heated to a plastic oreven molten condition suitable for adherence to the base metal. Althoughthe temperature within such a bed may well be considerably above thesafe treatment temperature for low carbon steels, the rapid movement ofthe strip through the bed limits exposure to such a short time that thesteel may be coated without serious overheating.

By this method, semirefractory and other inorganic and insulating-typematerials may be satisfactorily applied to steel strip. Coatings similarto those now used as separating media or core-plate coatings onelectrical steel may now be applied continuously and rapidly.

Many coating materials (of which Teflon plastic is an outstandingexample) are seriously degraded by heating to temperatures necessary forsatisfactory adhesion to the metal and cannot be properly applied byconventional coating methods. Other coating materials, e.g.,polyurethanes, are produced by instantaneous chemical reaction,preferably in direct contact with the base metal.

By permitting room temperature deposition of the thermoplastic materialsor through the employment of successive deposition of reactivecomponents, the electrostatic induction feature of my process permitsthe use of new coating materials that, because of their thermal orchemical properties, have not heretofore been appli- Although I havedisclosed herein the preferred embodiment of my invention, I intend tocover as well any change or modification therein which may be madewithout departing from the spirit and scope of the invention.

I claim:

1. A method of coating elongated metal product which consists in drawingthe product through a mass of particles of refractory coating material,fluidizing said mass by supplying a combustible mixture of gases theretoand igniting said mixture while passing through said bed hereby heatingthe particles of coating material to promote their adherence to theproduct.

2. A method of coating elongated metal product which consists in drawingthe product through a mass of particles of coating material, fluidizingthe mass by the upward flow of gas therethrough while maintaining anelectrostatic field between the product and the particles of the massadjacent thereto, and heating the product at a point in its travel to atemperature at which the particles remain adherent thereto.

3. Apparatus for coating a continuous metal product comprising a coatingchamber, a gas-permeable bottom plate in said chamber, said plate havingan opening to admit said product, guide rolls above and below saidchamber whereby the product may be drawn upwardly through said openingand a mass of coating particles in said chamber, means above saidchamber for heating the product after it has traversed said means, meansfor introducing a gas below said plate to fluidize said mass, anelectrode in said chamber spaced from the path of the product and meansestablishing a voltage gradient between said electrode and said product.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENC QH Checkel: Modern Plastics, vol. 36,. No. 2, October 1958,pages 125, 126, 128, and 132 (pages 130 and 132 relied on).

1. A METHOD OF COATING ELONGATED METAL PRODUCT WHICH CONSISTS IN DRAWINGTHE PRODUCT THROUGH A MASS OF PARTICLES OF REFRACTORY COATING MATERIAL,FLUIDIZING SAID MASS BY SUPPLYING A COMBUSTIBLE MIXTURE OF GASES THERETOAND IGNITING SAID MIXTURE WHILE PASSING THROUGH SAID BED